This invention relates to a positive type light-sensitive lithographic printing plate (or a positive type presensitized plate) and a development processing method of the positive type light-sensitive lithographic printing plate.
A light-sensitive lithographic printing plate comprises a light-sensitive layer provided on a hydrophilic support. For example, in a positive type light-sensitive lithographic printing plate, an ink-receivable light-sensitive layer which is made, soluble by exposure of active light such as UV ray is formed on a hydrophilic support.
In such a positive type light-sensitive lithographic printing plate, when a light-sensitive layer is subjected to image exposure and then the plate is developed, a portion exposed to light of the light-sensitive layer which is ink-receivable is removed to expose the surface of the hydrophilic support, while the ink-receivable light-sensitive layer which is not subjected to exposed to light remains on the support to form an ink-receivable layer. In lithographic printing, there is utilized such a difference in properties that the portion exposed to light is hydrophilic and the portion which is not exposed to light is lipophilic.
In general, a light-sensitive composition containing a o-quinonediazide compound as a light-sensitive component and an alkali-soluble resin as a component of heightening film strength and alkali-soluble property have been used in a light-sensitive layer of a positive type light-sensitive lithographic printing plate.
As the above o-quinonediazide compound, a 1,2-naphthoquinone-2-diazide-5-sulfonate compound and a 1,2-naphthoquinone-2-diazide-4-sulfonate compound have been used as useful ones. Particularly preferred is an ester compound of a polycondensed resin having a o-quinonediazide group such as a polycondensed resin of pyrogallol with an aldehyde or an ketone.
When an ester compound with a polycondensed resin is used as the o-quinonediazide compound, a polycondensed resin having a weight average molecular weight of 3,000 or more has been generally used for the purpose of improving press life.
As to development, there has been used a method in which development is carried out by replenishing a developing replenishing solution in order to replenish a developing solution component consumed by development processing.
When a positive type light-sensitive lithographic printing plate using a o-quinonediazide compound as described above is developed while replenishing a developing replenishing solution, a large amount of a developing replenishing solution is replenished so that a developing solution is in a fatigued state. Under such a state, when the light-sensitive lithographic printing plate is developed by using a step tablet (generally each concentration difference of 0.15, gray scale of 21 grades) for measuring sensitivity as a manuscript, a phenomenon that a portion corresponding to an intermediate tone portion of the step tablet of the lithographic printing plate becomes red is caused, and also a phenomenon that a red portion cannot be erased by a generally used erasing solution (correcting solution) is caused. In practical operation of processing a lithographic printing plate, these phenomena cause a problem that a portion corresponding to an edge portion of a film which is a manuscript frequently remains on a printing plate as an unnecessary image portion and the unnecessary image portion cannot be erased. Thus, there is a problem that a portion corresponding to a film edge portion which cannot be erased is inked to cause printing failure.
The above problem remarkably occurs particularly when a positive type light-sensitive composition is provided on a support subjected to graining treatment and anodization treatment as in a common light-sensitive lithographic printing plate and further remarkably occurs when an ester compound of a polycondensed resin is used as a o-quinonediazide compound.
It has been known that in order to solve the above problem that an intermediate tone portion becomes red, that is, a problem that a portion corresponding to an edge of a film which is a manuscript cannot be erased, a method of lowering a molecular weight of a polycondensed resin or adding a large amount of an organic acid or an inorganic acid to a light-sensitive layer is used.
However, in the above method for solving the problem, there is a problem that press life is lowered or the like, and further improvement has been demanded.
As a method for processing a light-sensitive lithographic printing plate in which developability is good even when repeated development processings are carried out for a long period of time and also precipitates (sludge) are not generated in a developing solution bath, there have been proposed a method of carrying out development by replenishing a replenishing solution comprising a solution of an alkali metal silicate having [SiO.sub.2 ]/[M] (where [SiO.sub.2 ] represents a molar concentration of SiO.sub.2 and [M] represents a molar concentration of an alkali metal) of 0.15 to 0.5 and a SiO.sub.2 concentration of 1.0 to 3.0 % by weight based on the total weight of the replenishing solution in Japanese Provisional Patent Publication No. 2273/1993 and a method of providing a back coat layer on the back surface of a support to which graining treatment is subjected in Japanese Provisional Patent Publication No. 35174/1994. However, even when a plate is made by using the development method or light-sensitive lithographic printing plate described above, a phenomenon that an intermediate tone portion of a developed light-sensitive lithographic printing plate becomes red is caused and a red portion cannot be erased. That is, a problem of erasing failure of a film edge portion is caused. Red color at the intermediate tone portion can be reduced by lowering a SiO.sub.2 concentration of a developing solution and a developing replenishing solution, but when the SiO.sub.2 concentration is lowered, there is a problem that precipitates in a developing solution bath are increased. Further, by providing a hydrophilic layer on a support subjected to graining treatment of a light-sensitive lithographic printing plate, erasing failure of a film edge portion and generation of precipitates in a developing solution bath can be controlled to some extent, but the effects are small and insufficient.